1. Field of the Invention
The present invention relates to an image heating device for an image forming apparatus which adopts an electrophotographic recording method or an elestrostatic recording method.
2. Description of the Related Art
Conventionally, heat-roller-type devices are widely used as thermal fixing devices applied to image forming apparatuses which adopt an electrophotographic or an electrostatic recording method. Thermal fixing methods that use thin films in which a toner image on a recording material is fixed by providing a film between a stationary heater unit and a pressing roller are known. With such devices it is not necessary to supply a heating fixing device with electric power in a standby state and they are able to minimize electric power consumption. Thin film fixing devices have been proposed, for example, in Japanese Patent Laid-Open Application (Kokai) Nos. 63-313182 (1988), 2-157878 (1990), 4-44075 (1992) and 4-204980 (1992).
FIGS. 7, 8(a) and 8(b) illustrate a device according to the film heating method which constitutes a background technique of the present invention.
FIG. 7 is a schematic diagram illustrating the configuration of a principal portion of a thermal fixing device according to the film heating method. FIGS. 8(a) and 8(b) are schematic diagrams illustrating the configuration of a heater of the device shown in FIG. 7.
As shown in FIG. 7, a stay holder 102, serving as a supporting member for supporting a heater 101 and guiding a fixing film 103, is provided. The heater 101, serving as a heating member, is fixed and supported on the stay holder 102. The fixing film 103 comprising a heat-resistant thin film is provided around the stay holder 102. The heating fixing device is configued by pressing an elastic pressing roller 110 against the heater 101 via the fixing film 103, to form a nip portion (fixing nip portion) N having a predetermined nip width. By passing a recording material P through the nip portion N, a toner image t on the recording material P can be fixed by being pressed and fused.
The stay holder 102 is made, for example, of a heat-resistant plastic material. The stay holder 102 holds the heater 101, and also operates as a conveying guide for the fixing film 103.
The heater 101 generally comprises a ceramic heater. For example, the heater 101 is configured by forming a current-passing heating resistive layer 101b, made of silver/palladium (Ag/Pd), Ta2N or the like, on a surface of a ceramic substrate 101a, which has an excellent electric insulating property, an excellent thermal conductivity and a low heat capacity and which is made of alumina or the like, facing the fixing film 103 in the longitudinal direction of the substrate 101a according to screen printing or the like, and providing a glass protective layer 101c covering the surface of the heating resistive layer 101b.
By passing current through the current-passing heating resistive layer 101b, the current-passing heating resistive layer 101b is heated, so that the temperature of the entire heater 101 including the ceramic substrate 101a and the glass protective layer 101c is rapidly raised. The temperature rise of the heater 101 is detected by temperature detection means 104 (see FIG. 8(b)) provided on the back surface of the heater 101, and the result of the detection is fed to a current control unit (not shown). The current control unit controls current supply to the current-passing heating resistive layer 101b so that the temperature of the heater 101 detected by the temperature detection means 104 is maintained at a predetermined substantially constant temperature (fixing temperature). Thus, the heater 101 is heated and controlled to the predetermined fixing temperature.
The fixing film 103 disposed around the stay holder 102 is in the shape of a cylinder, an endless belt, or a rolled web having finite ends, and is conveyed in the direction of an arrow "a" while slidably contacting the surface of the heater 101 at the fixing nip portion N, by driving means, or the rotational force of the pressing roller 110.
In order to efficiently provide the recording material P, serving as a material to be heated, with the heat of the heater 101 at the fixing nip portion N, the fixing film 103 has a thickness of 20-70 .mu.m and therefore is considerably thin. The fixing film 103 has three layers, i.e., a film base layer, a primer layer and a releasing layer. The film base layer faces the heater 101, and the releasing layer faces the pressing roller 110.
The film base layer of the fixing film 103 is made of polyimide, polyamideimide, PEEK or the like which is more insulating than the glass protective layer 101c of the heater 101, and which has a heat-resistive property and high elasticity. The film base layer provides the mechanical strength, such as the tearing strength or the like, of the entire fixing film 103. The primer layer is a thin layer having a thickness of about 2-6 .mu.m. The releasing layer is a layer for preventing toner offset with respect to the fixing film 103, and is made of a fluororesin, such as PFA, PTFE (polytetrafluoroethylene), FEP or the like, having a thickness of about 10 .mu.m.
In the heating device of the film heating method using such a thin fixing film 103, the pressing roller 110 having an elastic layer 111 is flattened at a pressed portion so as to follow the flat lower surface of the heater 101 due to high rigidity of the ceramic heater 101, to form the fixing nip portion N having a predetermined width. The fixing nip portion N is heated by the heater 101, so that quick-starting thermal fixing is realized.
When the recording material P, serving as the material to be heated, bearing an unfixed toner image t formed on a surface thereof is guided between the fixing film 103 and the pressing roller 110 at the fixing nip portion N in a state in which the heater 101 is heated and controlled at a predetermined temperature and the fixing film 103 is conveyed in the direction of the arrow "a", the recording material P is conveyed together with the fixing film 103 through the fixing nip portion N in a state in which the recording material P is in close contact with surface of the fixing film 103. The recording material P and the toner image t are heated by the heater 101 via the fixing film 103 at the fixing nip portion N, so that the toner image t is heated and fixed on the recording material P. A portion of the recording material P passing through the fixing nip portion N is peeled from the surface of the fixing film 103 and is conveyed.
FIGS. 8(a) and 8(b) illustrate the positional relationship between the current-passing heating resistive layer 101b of the heater 101, and the pressing roller 110. As shown in FIG. 8(a), the width W of the current-passing heating resistive layer 101b of the heater 101 in the longitudinal direction is slightly smaller than the width D of the elastic layer 111 of the pressing roller 110 contacting the current-passing heating resistive layer 101b via the fixing film 103, and is slightly larger than the conveying region of the recording material P bearing the toner image t formed thereon.
According to this configuration, the heat generated by passing current through the current-passing heating resistive layer 101b of the heater 101 is supplied to the recording material P as it is conveyed between the fixing film 103 and the pressing roller 110, to fuse the toner image t on the recording material P. The toner image t is solidified to form an image on the recording material P. Reference numeral 106 represents AC electrode portions.
As shown in FIG. 8(b), a temperature detection device 104, such as a thermistor or the like, and a thermoprotector 105, such as a temperature fuse, a thermostat or the like, for disconnecting current supply to the current-passing heating resistive layer 101b of the heater 101 during runaway are provided so as to contact the back surface of the heater 101 within the conveying region for a minimum-width recording material P which can be conveyed in the image forming apparatus. Reference numeral 107 represents DC electrode portions.
The temperature detection device 104 is provided within the minimum conveying region of the recording material so as to heat and fix the toner image t on the recording material P at an appropriate fixing temperature without causing problems, such as a failure in fixing, high-temperature off-set and the like, even when the minimum-width recording material P which can be conveyed in the main body of the image forming apparatus is conveyed.
The thermoprotector 105 is provided within the minimum conveying region of the recording material so as not to cause problems, such as disconnection of current supply, and the like, by misoperation of the thermoprotector 105, even during ordinary conveyance, due to overheating in the nonconveying region, having a smaller heat resistance than the conveying region, when the minimum-width recording material P is conveyed.
In the above-described heating fixing device of the film heating method, in order to improve the quick starting property, the thickness of the ceramic substrate 101a is minimized for the purpose of minimizing the necessary heat capacity of the heater 101. In this configuration, if the response of the thermoprotector 105 is not so good, there is the problem that, when the control unit for controlling current supply to the current-passing heating resistive layer 101b of the heater 101 or a safety circuit fails, i.e., when current supply to the current-passing heating resistive layer 101b runs away, the heater is, in some cases, destroyed due to overheating of the heater 101 and from the pressure provided in order to form the nip with the pressing roller 110 before the thermoprotector 105 disconnects current supply to the current-passing heating resistive layer 101b.
At that time, if only an AC supply portion of the heater 101 is interrupted, there is no electrical problem because current supply is disconnected, although the problem of destruction of the heater 101 is present. However, if a DC supply portion starting from the temperature detection device 104 is interrupted, there is the possibility that an AC supply portion and the DC supply portion short-circuit, resulting in, in the worst case, destruction of an electronic-component unit of the main body of the image forming apparatus. In order to solve such a problem, a configuration has been devised in which, as shown in FIG. 8(a), a through hole 101d is formed at a portion other than the DC supply region on the ceramic substrate 101a so as to assuredly interrupt only the AC supply portion due to stress concentration generated at the through hole 101d by heat and pressure. This configuration, however, causes problems because manufacturing the heater 101 is complicated, for example, by providing the through hole, and the production cost increases due to an increase in the number of processes.